1. Field of the Invention
The present invention relates to a sheet feeder for feeding paper sheets, and, more specifically to a sheet feeder for feeding copying paper to a developing unit of a copying machine.
2. Description of the Prior Art
In a sheet feeder of this type, a plurality of paper sheets previously stored in a sheet cassette are fed one by one to a photosensitive body, as a developing unit, during a copying operation, so that a copied image is formed on each paper sheet. Such a sheet feeder comprises a paper-supply roller for taking the paper sheets out of the sheet cassette, and a pair of aligning rollers for lining the sheets up before they are delivered to the developing unit. After being taken out of the sheet cassette by the paper-supply roller, each paper sheet is temporarily stopped by the aligning rollers to have its leading edge lined up. After the leading edge of the paper sheet is lined up, the aligning rollers are driven so that the sheet is delivered to the developing unit.
In the conventional sheet feeder constructed in this manner, the paper-supply roller and the aligning rollers are driven independently, and are alternatively started or stopped. According to this sheet feeder, however, the paper-supply roller and the aligning rollers are driven by a single, common motor. Thus, the sheet feeder unit is provided with a drive mechanism actuated by a single motor; and the paper-supply roller and the aligning rollers are fitted individually with clutches which can be coupled to the drive mechanism. As the clutches are alternatively coupled to the drive mechanism, the driving force is transmitted to either the paper-supply roller or the aligning rollers. Conventionally, electromagnetic clutches or spring clutches (one-way clutches) have been used for this purpose. Electromagnetic clutches, however, are expensive and may sometimes be subject to slippage in a connected state, attributable to the wear of clutch plates, while with spring clutches, a spring is normally in sliding contact with the driving-side shaft of the clutch such that it is liable to wear or fatigue, resulting in a diminishment of its resiliency. In some cases, therefore, spring clutches may fail to transmit the driving force when connected.
Thus, the driving force of the motor cannot be securely transmitted to the paper-supply roller and the aligning rollers, so that the paper sheets cannot be securely fed from the sheet feeder. This would necessitate the use of more expensive clutches, resulting in increased cost of the sheet feeder.